1. Field of the Invention
This invention relates to a dispersion compensator and an optical amplifier, and more particularly to a dispersion compensator and an optical amplifier which are free from an influence of polarization mode dispersion.
2. Description of the Related Art
In recent years, owing to an appearance of an erbium-doped optical fiber amplifier (EDFA), investigations have been and are being made for a transmission system which amplifies light directly. In such circumstances, a demand for increase in capacity, increase in transmission distance and improvement in function wherein an EDFA is employed is increasing also in a single mode fiber network which exhibits a zero dispersion in the 1.3 .mu.m band of an existing system. Since an EDFA is an optical amplifier which operates in the 1.5 .mu.m band, where it is applied to a single mode fiber network which exhibits a zero dispersion in the 1.3 .mu.m band of an existing system, it transmits an optical signal of the 1.5 .mu.m band therethrough, and in this instance, an influence of color dispersion(wavelength dispersion) cannot be ignored. Accordingly, in order to make such application possible, a dispersion compensator for suppressing color dispersion is required.
One of popular methods of suppressing a color dispersion is to connect a dispersion compensator which has dispersion of a sign opposite to that of dispersion produced in a transmission line to an intermediate portion or an end of the transmission line so as to offset the color dispersion of the transmission line. Various dispersion compensation have been proposed including (1) a dispersion compensator which employs a grating, (2) another dispersion compensation which employs an optical interference unit and (3) a further dispersion compensator which employs an optical fiber (dispersion compensation fiber). Among various dispersion compensators proposed by now, a dispersion compensator which employs a dispersion compensation fiber is expected to be highest in practicability due to the facts that it does not require a control circuit and so forth for stabilizing the operation of the dispersion compensator and allows a passive operation and that it has an applicable wavelength bandwidth much wider than the other compensators.
Since a single mode fiber employed for a transmission line has color dispersion of the positive sign of approximately +15 to +20 ps/nm/km in the 1.5 .mu.m band, a dispersion compensation fiber is required to have color dispersion of the negative sign. Further, in order to manufacture a dispersion compensator in a small size, the color dispersion of the dispertion compensation fiber is required to have a high absolute value. In order to satisfy those requirements, various solutions have been proposed including adoption of a structure which has a very high numerical aperture (M. Onishi, Y. Koyama, M.Shigematsu, H. Kanamori and M. Nishimura, "Dispersion compensating fibre with a high figure of merit of 250 ps/nm/dB", Electron. Lett., 1994, vol. 30, no. 2, pp.161-163), adoption of a multiple clad structure (Ashish M. Vengsarkar and W. A. Read, "Dispersion-compensating single-mode fibers: efficient designs for first- and second-order compensation", Opt. Lett., 1993, vol. 18, no. 11, pp.924-926) and adoption of an elliptic core structure (C. D. Poole, J. M. Wiesenfeld and D. J. DiGiovanni, "Elliptical-Core Dual-Mode Fiber Dispersion Compensator", IEEE Photon. Technol. Lett., 1993, vol. 5, no. 2, pp.194-197), and dispersion values exceeding -100 ps/nm/km have been reported.
Whichever technique is employed, in order to realize a dispersion compensation fiber of a high performance, the core diameter is reduced to a very low value comparing with an ordinary single mode fiber or the structure is complicated. A fiber of the type just mentioned is liable to exhibit some non-uniformity in structure, and there is the possibility that the polarization mode dispersion may have a considerably high value such as, for example, 1 ps/.sqroot. km or more comparing with the polarization mode dispersion (for example, 0.1 to 0.2 ps/.sqroot. km) of an ordinary single mode fiber. As an example, if a transmission system of 200 km which employs as a transmission line a single mode fiber whose dispersion value with respect to an optical signal of the 1.5 .mu.m band is 18 ps/nm/km is considered, then the dispersion which is produced in the transmission line is 3,600 ps/nm. In order to cancel this dispersion using a dispersion compensation fiber having a dispersion value of -100 ps/nm/km, a dispersion compensation fiber of the length of 36 km is required. Where the dispersion compensation fiber has polarization mode dispersion of 2.5 ps/.sqroot. km, relying upon the polarization condition of light incident to the dispersion compensation fiber, a delay time difference of approximately 2.5.times..sqroot. 36=15 ps is produced over the distance of 36 km. This value is approximately 15% of 100 ps which is one time slot in transmission at the rate of 10 Gb/s and makes a factor which deteriorates the reception sensitivity very much. Meanwhile, also a doped fiber of an EDFA is liable to produce polarization mode dispersion due to its non-uniformity in structure, and it is required to prevent such polarization mode dispersion.